1. Field of the Invention
The present invention relates to an anisotropically electroconductive connecting material which is suited to the manufacture of liquid crystal panels which use plastic substrates.
2. Description of the Related Art
Liquid crystal panels which use liquid crystal cells, in which a liquid crystal is sealed between a pair of transparent glass substrates, have been used in electronic devices such as mobile telephones and laptop computers, which have been enjoying widespread use.
These liquid crystal panels are produced by sandwiching an anisotropically electroconductive connecting material having electroconductive particles dispersed in a thermosetting insulating adhesive, between indium-tin-oxide (ITO) electrodes on a glass substrate of a liquid crystal cell and connection terminals on a flexible substrate, then thermocompression bonding the assembly to effect mutual adhesion therebetween while maintaining the electroconductive connection therebetween.
Recent attempts to make these liquid crystal panels lighter and thinner have involved substituting transparent glass substrates of the liquid crystal cell with transparent plastic substrates such as polycarbonate substrates, polyether sulfone substrates, which transparent plastic substrates are lighter and stronger than the transparent glass substrates.
However, these plastic substrates have a softening point at 100 to 220xc2x0 C., unlike glass substrates, and this softening point overlaps the compression bonding temperature range. Accordingly, stresses build up on the ITO electrodes which contact the electroconductive particles during compression bonding, so that, the portion of the plastic substrate affected suffers deformation and cracks appear in the ITO electrodes, resulting in a poor display. This drawback is particularly evident when the electroconductive particles used can enable good connection reliability in case of manufacturing liquid crystal panels which use glass substrates (i.e., electroconductive particles which have a high modulus of elasticity and which do not soften during compression bonding).
Measures used to counteract this problem have involved using electroconductive particles comprising a resin core on the surface of which a metal layer has been formed, which resin core deforms under relatively light loads. However, due to the fact that the electroconductive particles suffer plastic deformation, heat, external stresses, humidity and other factors cause the dimensions of the connection portions to change, and the particles cannot follow these changes in such cases; post-ageing connection reliability suffers an especially marked deterioration.
The drawbacks described above occur not only during the manufacture of liquid crystal panels, but also when substrates are used which deform readily under thermocompression bonding conditions in case of establishing anisotropically electroconductive connections, and also in case of using electroconductive particles which are plastically deformable.
With the foregoing problems of the prior art in view, it is an object of the present invention to provide an anisotropically electroconductive connecting material which allows the ITO electrodes not to crack, or the post-ageing connection reliability not to deteriorate, when establishing anisotropically electroconductive connections between mutually opposing substrates, even when a plastic substrate on which ITO electrodes have been furnished is used as the substrate,
The present inventors perfected the present invention by discovering (1) that by keeping the modulus of elasticity at the compression bonding temperature of the electroconductive particles used in the anisotropically electroconductive connecting material at a pregiven value or less than the modulus of elasticity at the compression bonding temperature of the substrate to be connected, even when a plastic substrate on which ITO electrodes have been furnished is used as the substrate, it is possible to avert any cracking in the ITO electrodes; and moreover (2) when the modulus of elasticity within the normal temperature range (e.g., 25xc2x0 C.) of the electroconductive particles is raised higher than the modulus of elasticity of same at the compression bonding temperature, then post-ageing connection reliability can be enhanced.
In other words, the present invention is an anisotropically electroconductive connecting material, which is provided in order to avert any cracking in ITO electrodes when a plastic substrate on which ITO electrodes have been furnished is used as the substrate, which is disposed between connection terminals on a first substrate and connection terminals on a second substrate, and joins the substrates together by means of thermocompression bonding while maintaining the electroconductive connection therebetween, and which comprises electroconductive particles have been dispersed in an insulating adhesive, wherein the modulus of elasticity of the electroconductive particles at the compression bonding temperature is 200% or less of the modulus of elasticity of the first substrate at the compression bonding temperature.
It is preferable for the modulus of elasticity of the electroconductive particles at the compression bonding temperature to be 60 to 90% of the modulus of elasticity of same at 25xc2x0 C., in order to further impart properties which can enhance the post-ageing connection reliability in the aniostropically electroconductive connecting material.